Apparatus for heat treating gears and the like



g- 1, 1939- F. s. DENNEEN ET AL ,167,798

APPARATUS FOR HEAT TREATING GEARS AND THE LIKE Filed Aug. 19, 1935 3 Sheets-Sheet 1 VENTORJ Aug. 1, 1939.

F. S. DENNEEN El AL APPARATUS FOR HEAT TREATING GEARS AND THE LIKE Filed Aug. 19, 1935. 3 Sheets-Sheet 2 INVENTORJ 1, 1939- F. s. DENNEEN ET AL 2,167,798

APPARATUS FOR HEAT TREATING GEARS AND THE LIKE Filed Aug. 19, 1935 3 Sheets-Sheet 3 Pam-wane 1, 1939. 2,167,798

UNlTED STATES PATENT OFFICE APPARATUS FOR HEAT TREATING GEARS AND THE LIKE Francis S. Denneen, Cleveland, and William 0.

Dunn, Shaker Heights, Ohio, assignors to The Ohio Crankshaft Company, Cleveland, Ohio, a

corporation of Ohio Application August 19, 1935, Serial No. 36,843

11' Claims.

The present invention, as a further improvement on that shown and described in our copending application Serial No. 689,904, filed September 18, 1933, relates, as indicated, to apparakept cool in order to preserve the strength imparted by, previous treatments. Further, the products of combustion have a harmful elfect o the surface metal.

6 tus for hardening gears and the like. Whereas, No previous method has been provided whereby the former application refers generally to means only such parts of the gears as are required to and method for electrically heating metal articles be hardened are heated, while other parts are to be quenched to provide hardened surfaces maintained cool enough that previous heat treatthereon, the present application is more specific ments imparted to provide toughness and resistin its scope, being confined to more definite and ance to shock remain substantially unaffected. detailed uses of the general process taught in the Our method is applicable not only to gears, but above numbered application. to numerous other articles as hereinafter ex- In the heat treatment of metals generally and plained, such as splined shafts, reamers, drills, in the hardening of ferrous metals in particular, gun bores, the wearing surface of pulverizing and there have been no teachings which would enable crushing machines, friction feed rolls and the u those skilled in the art to produce hardened zones like, and to almost any metal article requiring a of metal of predetermined characteristics as to hardened zone, particularly where there is an undepth and general contour on articles having even surface. abrupt or irregular changes in surface form by With the aforementioned conditions in view,

the use of electric heat. Surface hardening procthis invention then has as its chief object the 20 esses used heretofore, were not adapted to the provision of a means whereby the wearing surhardening of such surfaces as the contacting faces of a gear may be hardened without detrisurfaces of gear teeth or splines, the side walls mental changes affecting other parts of the gear. of sliding keys and keyways, the rifled interiors Another object is to provide means whereby the of gun bores, and many other similar surfaces. addendum portions of all teeth may be hardened 25 In the hardening of such surfaces as those above simultaneously and uniformly. A further object enumerated, it is often important that adjacent is to provide means whereby a portion of a gear surfaces as well as the core or metal under the after being hardened may be tempered to any surface, be kept softer in order that it be suiildesired degree. A still further object is to prociently ductile to resist fractures due to shock, vide means whereby internal teeth or splines 80 vibrations and other causes. For instance, in a may be hardened simultaneously with the hardgear tooth, the addendum surfaces should be hard ening of external teeth on the same gears. Anin order to resist the abrasion of the sliding and other object is to provide means whereby the disrolling contact of other tooth surfaces, while the tribution of heating over a surface may be con- 85 metal in the dedendum and at the root of the trolled. An additional object is to provide means 86 tooth which does not contact with any wearing which is rapid, accurate and applicable to a or ab a ve element, but ch is Su mi e to variety of parts. A still further object is to proheavy bending stresses due to the cantilever conid apparatus for hardening gears which may be struction of ar teeth, should remain s r n n rapidly applied to the gear and then removed comparatively ductile to p n breakage or the therefrom. A further object is to provide heat- 40 formation Of incipient fractures. Experience has ing apparatus which serves also as means for detaught that brittleness in steel usually increases nvering a quenching medium to the heated with hardness, and the greater the brittleness mew An additional Object is to provide means h more the metal is suscgpflble to the forma for producing an article having localized hardtron of surface cracks or incipient fractures which ened wearing surfaces Another object is to 45 eventuauyextend far enough into the metal to vide means whereby the internal surfaces of bores, cause complete failure of the section affected.

- such for instance as in fire arms, cannon or Heretofore, in methods of hardemng gears, the I 1 h th m gears have either been heated to the required other artillery p Bees W 9 er smoo temperature as a whole and the entire gear then bored fled, may be hardened' Numerous 50 Y quenched resulting in Shrinkage strains and other ob ects will become apparent from the folwarpmg or the teeth have been heated mmlowing description in which details of apparatus viduany by applying a flame t t d capable of producing improved results will be quenched. With the latter method, the heat is defined and new and advantageous steps in the 66 boundto penetrate into regions that should be procedure for obtaining such results will be described or suggested to those skilled in the art of heat treating.

In the annexed drawings:

Fig. 1 illustrates one form of apparatus and shows a sectional view of a gear in operating relationship within the apparatus. Fig. 2 is an elevation, partly in section on line IIII of Fig. 1. Fig. 3 which is a section similar to that of Fig. 1 but taken on line III--III of Fig. 4 illustrates a modification adaptable to a particular type of gear. Fig. 4 is an elevation partly in section on line IV-IV of Fig. 3. Fig. 5 is an illustration, partly in section on line V-V of Fig. 6 of a modification of .the apparatus adapted for the heat treatment of internal gears or splines. Fig. 6 is a section on line VI-VI of Fig. 5. Fig. 7 is a sectional view showing a still further modification of the apparatus as applied to a spur gear. Fig. 8 is a fragmentary view substantially as indicated at VIII-VIII of Fig. 7 with one spacer omitted to expose to view a part of the root section of the. gear. Fig. 9 is an elevation of a gear in the apparatus substantially as shown in Figs. 1 and 2 but having other means for distributing heat. Fig. 10 is a fragmentary section on line X-X of Fig. 9. Fig. 11 illustrates a modification of the mechanism shown in Figs. 1 and 2. Fig. 12 illustrates means for removing conducting fillers. Fig. 13 is a view of the apparatus as shown in Fig. 4 but on a reduced scale. Fig. 14 illustrates diagrammatically the relative positions of internal and external inductors. Fig.

, 15 is a vertical sectional view of apparatus for heating and quenching the internal splines of the gear shown in Fig. 3 simultaneously with the hardening of the surfaces of the external teeth. Fig. 16 shows the expander for releasing the spacers shown in Fig. '7.

While in most of the accompanying drawings and descriptions the preferred method of heating is by high frequency induced currents set up in the zones to be hardened, the heating may also be accomplished by the direct flow of current into the article through electrical contacts established between the article and parts of the apparatus as is illustrated in our co-pending application Serial No. 689,904 above referred to.

When an irregular shaped article such as a gear or splined shaft is placed within an inductor coil supplied with high frequency current, the induced currents in the gear tend to concentrate in the first continuous circular paths just below the teeth thereby heating the part of the gear below the teeth rather than the teeth themselves. We are showing herein various means whereby the induced currents may be caused to fiow in the teeth themselves, thereby heating them preparatory to quenching. Means are also shown whereby heat may be generated by induced currents set up in material adjacent to the teeth in such a way that the heat is evenly and rapidly communicated particularly to the' top and side portions of the teeth without serious injury to the strength or other physical properties of the portions of the article adjacent to the heated zones.

Referring now more particularly to Figs. 1 and 2, the gear 20 may be supported by a shaft or arbor.as illustrated in Fig-12'so as to be held concentrically. in the inductor 22. This, inductor, whichis substantially a single coil for high frequency current surrounding the gear 20, comprises the inner cylindrical wall 23, in which the current tends to concentrate, and side walls 24 and 25. The outer or Jacket portion 26 is preferably insulated therefrom by the insulation 27. Electric current preferably of high frequency is supplied thru terminals 28 and 29. Since the current induced in the gear tends to flow parallel with, but in a direction opposite to the greater part of the current in the inductor, the current in the gear will follow a substantially circumferential path at the roots of the teeth and in so doing will thus heat the dedendum parts of the teeth more rapidly than the addendum parts. As it is desired to heat the addendum only and refrain from heating the dedendum parts, contacting fillers 30, which may be 'made from material with substantially the same magnetic qualities as the gear teeth or from other conducting metal having heat resisting qualities such as nickel chrome, ferrous alloys or other suitable metals, are-brought into conducting relationship with the addendum faces of the teeth, thus forming a substantially continuous circumferentially conducting path in which the greater part of the induced currents flow. Heat will be set up in the fillers by the current induced in them and part of this heat will be communicated to the adjacent gear teeth, principally along the contacting surfaces. Since the contact between the fillers and the tooth faces will offer appreciable resistance to the fiow of the induced heating currents, there will be additional heating developed in the tooth surfaces along these faces of contact, thereby tending to set up greater heat at the tooth surfaces than in the deeper areas. These fillers are retained in firm contact with and between the teeth by bands 3| and 32 which may be either continuous or split and arranged with simple resilient means as shown at 29' to act compressively on the ends of the spacers 30 to offset the expansion of the bands as the heating progresses. The heat caused by current induced in the fillers 3|] and the addendum parts of the teeth expands these fillers, tending to maintain firm contact until the critical temperature is attained. The bands 3| and 32 are preferably insulated as at 33 from the fillers 30. Depending upon production requirements, the fillers 30 may be either mechanically removable, or fusible at, or a little above, the critical temperature of the material of the gear. At the termination of the heating interval, a vigorous quenching of the heated portions of the gear teeth is accomplished by projecting quenching fiuid onto the tooth surfaces at high pressure through the orifices 34 in the face of the inductor 22, the pressure being sufficiently high to deliver ample coolant and to remove any steam or gas formations from the surfaces to be cooled. The quench is supplied from the jacket space 35 to which it is delivered under pressure at the proper instant thru piping connected to the threaded member 36. Supplementary quenching is provided by additional coolant being projected from the nozzles 36' into the space between teeth.

In cases where it is necessary or desirable that leaving free access to the heated tooth surfaces for the quenching fluid. Tofacilitate assembling the fillers with the gear, the fillers and the ring member may be die cast together as an assembled unit from suitable fusible material so that all the fillers may be slipped in place between the teeth with a single operation, the free ends of the fillers being held in position by a compression ring. The insulating member 33, which is required in some cases, is omitted. This construction is illustrated in section in Fig. 11 in which the fillers 30' are formed integrally with the ring 32 and have their free ends compressed by the band 3| which is contracted by spring 20", this construction being similar to that shown in Fig. 2. Obviously, the apparatus is preferably set in horizontal position to permit, the melted filler material to drop away freely; The filler material so melted away is salvaged, remelted and used repeatedly.

Substantially equivalent results are, under certain conditions, accomplished by filling the space between the teeth with a plastic material composed of finely divided iron mixed with a suitable binder. A modification of this method is to prepare a filler mixture containing in addition to iron, some lead, copper, tin, antimony, or-any of a considerable number of fusible materials in varying combinations, so that when the teeth reach the desired heat for quenching, part of the constituents of the filler mixture melts, permitting the mixture to fall away from between the teeth, leaving free access for the quenching fiuid against the heated surfaces.

To aid in the removal of the filler material, a simple mechanism illustrated in Fig. 12 and consisting of a series of metallic fingers 3|", one for each tooth space; attached to a suitable ring 32", is arranged so that with a simple movement the fingers are simultaneously inserted to push out the fillers and then simultaneously withdrawn from between the teeth, thereby clearing the tooth spaces for the quench.

In using the plastic filler material, in some cases it is desirable to entirely fill the tooth spaces and in others there is advantage in having the filler material contact only the upper portions of the teeth, leaving the lower portion of the tooth spaces unfilled as shown in Fig. 2. As we have stated elsewhere, making contact with the upper portions of the teeth only, tends to confine the heating more to the tooth tops. By leaving the bottom portions of the spaces open and by arranging the quenching equipment to direct a. part of the quenching liquid under pressure into these open spaces, dislodgment and removal of the filler material is aided and the quenching materially speeded up.

For some purposes, certain advantages are obtained by baking ordrying the plastic material after being put in place between the teeth and before the heating operation, or by forming the filler material to shape, then drying or baking and inserting in place between the teeth before the heating operation.

Another modification of our process consists in coating either the gear teeth, or the fillers, or both, with a fusible alloy that melts when the proper temperature has been reached for the quenching, then assembling the fillers in position between the gear teeth, and then heating and quenching as elsewhere described. When this procedure is followed, the fusible coating forming the joint between the fillers and the teeth melts when the quenching temperature is reached and the fillers drop out, leaving free access for the quenching fluid against the heated'surfaces. The

fillers are inserted between the teeth and pressed into position with enough pressure to deform the fusible coatings suiliciently to hold the fillersfirmly in position until the fusible coating has melted. Usually there are enough irregularities on the surface of the teeth and the fillers to insure this locking action; however, slight serrations or deformations may be provided in one or both if necessary to accomplish this locking.

To aid in dislodging the fillers when the fusible coating has melted, the ring and finger assembly mechanism of Fig. 12 above described for removing plastic fillers from between the teeth, is used to advantage.

In the modification illustrated in Figs. 3 and 4 the current conducting filler elements 31 are all attached to a member to operate as a unit so that they may be simultaneously inserted into or removed from the tooth spaces of the gear 3! to be heat treated. The filler elements 31 are hinged to the ring 38 and insulated therefrom by the insulating bushings Ill and spacing washers on each side of the filler members as shown by broken lines at M. The ring 39 is centered on the shaft or arbor ll carrying the gear 38 to be treated. Rods 42 are provided for advancing and retracting the spacer element assembly. To prevent a radial spreading of the tips of the fillers, these are confined by a refractory and insulating ring "interposed between these tips and the inner part of the inductor 44 which surrounds the gear.

To obtain a better contact between the fillers and the tooth surfaces, thereby improving the conductivity of these contacts, the fillers are coated by painting, spraying, hot or cold dipping, electroplating or by other means with a compressible conducting coating such as a conducting paste, or plastic compound, or plastic metal, or metal alloy. The conductivity of the path for the heating currents is otherwise improved by polishing either or both the fillers and the teeth or coating or plating them with some suitable metal or alloy.

The inductor 44 is made in two parts, preferably connected by a current conducting hinge as shown at 42' in Fig. 13, and is thus adapted to be opened to receive the gear 38 and permit the portion 45 of the inductor to enter the groove 46 in the hub of the gear. As high frequency current in passing from the terminal 41 around the inductor tothe terminal 48 is confined mainly in the inner wall 49 of the inductor, and as the spacer elements 31 in contacting with the faces of the addendum portions of the teeth provide a continuous conductor, heating currents of high density are induced in the tips of the teeth and in the surfaces of the annular groove 46. As soon as these induced currents have raised the surface portions of the gear to hardening temperature, the current is interruptedand a quenching fiuid is introduced simultaneously from pipes 50 and ii into Jacket spaces 52 from where it is projected vigorously through orifices 53 onto the heated surfaces it is desired to harden.

For the hardening of elements having internal teeth such as annular gears, splined hubs, positive clutches, hollow milling cutters and the like, or gun bores, apparatus such as shown in Figs. and 6 is employed. In this illustration the splines 54 of the hub 55 represent the members to be'hardened. Contacting spacers 56 are inserted tightly in the tooth spaces in a manner somewhat similar to that employed in connection with the apparatus shown in Figs. 1 and 2. These spacers are held in assembled position by the grooved circular end rings 51 and it. The internal inductor 59 is then inserted and by suitable means is held in concentric spaced relationship with the inner faces of the teeth. High frequency current is supplied thru one or more terminals such as 60 at one side ofthe division SI of the inductor and the circuit is completed thru other terminals 62 at the other side of this division. Inducing current is thus caused to flow circumferentially in the inductor and induced currents, by virtue of the spacers 56, flow in the inner faces of the spindle teeth bringing these to hardening temperature with sufficient rapidity to prevent heat from migrating harmfully into the root sections. As soon as the desired temperature has been attained in the faces of the spline teeth, the heating current is interrupted and a vigorous quenching of the heated surfaces is provided by projecting a quenching fluid from the jacket space 63 thru passages 64 onto the heated surfaces. The quenching fluid is supplied by the pipe 65.

In case the face of aspur gear is wide or the teeth are small, the individual cantilevers 31. as shown in Figs. 3 and 4 would have insufficient inherent strength to maintain uniform current conducting contacts across the entire face of the gear tooth, and rapid production requirements would prohibit the handling of individual spacers as shown inf Figs. 1 and 2. To eliminate these diificulties, the spacers are arranged as shown in Figs. 7 and 8. In this embodiment the spacers 66 are disposed like the link pins of a chain. Flat links 61 perforated to receive the cylindrical ends 68 of the spacers are disposed at each end of the spacers and join all the spacers required for one gear into a single chain-like assembly which may readily be removed from one gear which has been hardened and applied to another. The links and spacers would be made from suitable heat resisting material.

To facilitate the handling of the link and spacer assembly a base 8| and cover 8| are provided with conical surfaces 69 and 10 respectively, so that when the cover is advanced toward the base, the pin members are pressed radially inward forcing the spacers into firm contact with the teeth, enough freedom being provided in the fit of the parts to permit this action. The shaft II by means of a guide bushing in which it is free to move ax ally serves to hold this cover in concentric relationship with the gear and the spring or other resilient member 12 brings this cover into pressure relationship with the links.

As the gear 13 with the spacer assembly engaging the teeth is placed in position on the arbor 14, it enters into concentric relationship with the inductor 15 which is similar in form to that shown in Fig. 2. High frequency heating current is supplied thru a pair of terminals, one of which is shown at 16, and quenching fluid is introduced into the jacket space H thru the fitting I8. At the proper instant after the addendum portions of the gear teeth have attained hardening temperature, due to currents induced by new of high frequency currents in the inductor, the quenching fluid is projected thru orifices 19 onto the tops of the teeth. Radial passages in the contacting faces of the spacers 66 permit a part of the quenching fluid to flow inwardly and along the surface of the teeth rapidly cooling the surface to be hardened.

In some cases it may be necessary to withdraw the spacers from contact with the gear teeth in order to permit freer and more rapid quenching of the heated tooth areas. When the proper quenching temperature has been reached, this is accomplished by relieving the pressure on the spring 12 and withdrawing the shaft H to lift of this fluid will enter passages 80 and such additional space as has been provided by the loosening of the spacers by spring 82' which is shown separately in Fig. 16. It will be evident that passages 19 may be so located as to bein alignment with passages 80 after these latter have beenraised by spring 82 and thus a part of the quench is projected directly into these latter passages, thereby enhancing the rate of cooling. As the pressure of the conical surfaces 69 and 10 is released from the pins, spring members preferably of flat stock shown at 82' and in Fig. 16 press radially outward against the links of the chain assembly, forcing the spacers radially outward from engagement with the gear teeth, leaving ample space for the flow of coolant between the spacers and the teeth. The chain-like spacer assembly can be quickly changed from one gear to another, permitting rapid production of hardened gears. This assembly contains just the right number of spacers to fill all tooth spaces. The spacer at one end of the assemly is placed in any selected tooth space and then the assembly is wrapped around the gear, each successive spacer dropping into a successive tooth space as shown in Fig. 18. The last spacer which is at the free end of the chain drops into the space just one tooth removed from the first spacer which is at the other end of the chain thus filling all tooth spaces and completing a circumferential path for the induced heating current.

In some instances, the spacers may be dispensed with and a'band, either of flat stock as shown at 83 in Figs. 9 and 10, or of wire, is applied to the periphery of the gear to be hardened. This band provides a continuous circuit around the gear for the flow of induced high frequency current. As shown in Figs. 9 and 10, the gear 84 with the band 83 in place thereon is inserted centrally in the inductor 85 which is supplied with inducing current thru the terminals 86 and 81. The currents induced in the band 83 rapidly bring this band to high temperature and the conduction of heat therefrom into the tops of the gear teeth together with any current that may be caused to flow in the teeth raises these teeth rapidly to hardening heat.

The band 83 is made of material adapted to withstand high temperatures, and certain ferrous alloys containing chromium or nickel. or manganese or combinations of these elements may be employed to advantage. As the temperature of the band rises, electric resistance increases, its permeability changes, and induced heating currents develop in the root section. of the gear which section should be kept cool. Therefore, after the band reaches a sufllciently high temperature, the power input to the inductor preferably should be maintained so as to prevent heating the root section by induced currents and the transfer of heat from the band to the tops of the teeth should be maintained at a rate which will bring the portions to be hardened up to hardening temperature before harmful heating occurs elsewhere.

Quenching fluid under high pressure is introduced into the jacket space 99 by a fitting applied at 89. This fluid is projected vigorously onto the band thru orifices 90 and between the teeth from a series of properly located nozzles 92 immediately after the termination of the heating interval. In some instances where more rapid quenching is necessary, the band may, at the instant of quenching, be quickly pulled off the teeth by one or more hook members 9|, previously in position, thus leaving a free, uninterrupted fiow'of quench onto the surfaces of the gear teeth, this quenching being supplemented by fluid projected from nozzles 92.

It will be evident that the apparatus as illustrated will suggest may advantageous modifications to those skilled in the art. For example, the dedendum and root section of the teeth of hardened gears may be tempered by the use of this apparatus if the contacting spacers between the teeth are omitted, whereby the heating current is caused to fiow circumferentially in the section at and below the roots of the teeth. The form and location of the teeth is such as to prevent current from flowing in the teeth themselves so that the teeth will remain substantially unheated except for the conduction of heat from the root sections.

It will be understood that the form of the fillers, their material, and the rates of heating and quenching may be such as to provide hardened surface zones in the addendum part of the teeth leaving a relatively soft core extending toward the tips of the teeth. The tooth surfaces closest to the inductor will be heated most rapidly and those quenched most rapidly at the critical temperature will be the hardest.

When gear teeth are subjected to surface impacts tending to break corners of the tooth tips as in certaintypes oi clutches, it is important that these tips be made somewhat less hard than the addendumfaces to prevent this breakage. This may be accomplished by locating the fillers deep enough in thevtooth spaces to confine the heating to approximately midway of the tooth height. In this 'way the temperature of the tooth tips may be kept appreciably below critical temperature so that there will be an annealing rather than a hardening action at these tips, although there will be hardening action slightly below the tips. Still another method would be to slightly temper or draw the tooth tips by annealing.

It is to be understood that the process herein described is not limited to gears andthe like made from forged or rolled steel, but may be applied to cast ferrous metals and to certain non-ferrous metal to produce qualities therein peculiar to that metal.

By our process of heat treating gear teeth, the heating and quenching is accomplished so uniformly that substantially no distortion remains upon completion. The heating being confined to the addendum portions of the teeth only contraction due to quenching is limited to the addendum and substantially no distortion takes place in the dedendum part of the teeth or in the rim section at or below the root, or in the web of the gear. Hence, the teeth are easily kept in the desired accurate final form without any grinding or other operations subsequent to hardening. If, however, final finishing after hardening is required, the hardened zone is 01. sufiicient depth to permit the removal of surface material without penetrating below the region of satisfactory hardness.

It is to be noted from the foregoing that hardening'heat is confined mainly to the surface zones of the addendum parts of the tooth and that a core of softer metal extends from .the root circle outwardly toward thetip of the tooth and the form and extent of this core depending upon the rate of heating applied, the form of the zone to which the heating is limited, the rate of quench and shielding or other method used in applying the quench. It will be evident that the resulting hardness will be different for different analyses of the material of the gears.

The circulation of a cooling fluid, such as air, thru the spaces between the contactors and the roots of the teeth willaid in the control of the rise 01' temperature in the dedendum part of the teeth. This isaccomplished by passing air of desired temperature and amount thru nozzles such as 92.01 Fig. 10 during heating, and then discharging water from these nozzles instead of air for the quenching interval.

It will be evident that an inductor similar to the one shown in Figs. 5 and 6 maybe inserted in the splined bore of the gear 31 of'Figs. 3 and 4 and that by successive applications from the same source of power or by simultaneous applications from the same or different sources of power, both the internal and external teeth may be hardened at the same setting of the gear in the apparatus. Such an arrangement is shown in Fig. 15 in which the gear 93, which is substantially the same as gear 38 of Fig. 3 is provided with internal splines or teeth 94 which are to be hardened at the time of hardening the outer teeth 95. The internal inductor 96 is substantially the same as inductor 59 of Figs. 5 and 6, excepting that it is made on a somewhat smaller scale to enter the spiined hole in the gear 93. This internal inductor is inserted in the gear with suitable clearance and is supported on shaft 91 with suitable insulation. The spacers 99 which are substantially identical with the spacers 31 of Figs. 3 and 4 are held in firm contact with the tooth surfaces by theengagement with outer inductor 99 thru insulation I00. These spacers being carried by a support ring I which corresponds with ring 39, oi. Fig. 3, are secured in a relatively rigid assembly with this ring and the gear by the inductor 99, the insulation I00 preventing the tips of the spacers from spreading and thus holding the sides of the spacers in firm contact with the sides of the gear teeth. The outer inductor 99 has a jacket space I02 for supplying quenching fluid thru orifices I 03 to the tops of the teeth and the inner inductor 96 likewise has a jacket space I04 to supply quench to the surfaces of the internal splines. Current is supplied to the inductors thru terminals I05 and I 06, the inductors being joined in series by the connection I01 in reverse current relationship or connected in parallel across power lines as shown diagrammatically.

In the preceding description and in the appended claims, reference to the addendum portion of a gear tooth refers to that portion radially outward of the pitch line or rolling surface. The dedendum portion refers to that part which is radially inward from the pitch line. The root of the tooth is the part of the gear to wh ch the dedendum part of the tooth is attached.

Reference to high frequency current in this specification implies current having a frequency considerably in excess of the frequency of ordinary commercial power current.

While in the foregoing description of our process we have throughout specifically referred to gears, it is understood that the process applies to the various articles mentioned herein, and, in addition, to a wide variety of others not specifically named.

In the appended claims the term "tooth flank of a gear defines the side of the tooth with which a mating tooth of another gear contacts. By gear blank" is to be understood the piece of material, straight or curved, on which teeth are cut or are otherwise provided as by casting, insertion of separate pieces, welding or by other processes.

The core of a gear tooth, or of another projection on a piece of heat treatable material, is understood to be that portion of the-tooth or projection sufilciently far below the surface of the metal as not to be affected by the heating and quenching and which therefore remains substantially in its original condition during the heating and quenching of an adjacent surface zone.

Groove" has been used to define any depression of whichthe space between two teeth is typical, but applies to any longitudinal depression which may exist in the surface of the article to be heat treated, such as in a splined shaft.

' frequently found that merely flowing the quench over the heated surface is insuificenit and that the quenching fluid must possess considerable kinetic energy in order to remove the insulating bubbles and vesicles while in their incipient form. This kinetic flow has been defined by the use of the term projection.

Other modes of applying the principle of our invention may be employed instead of the ones explained, change being made as regards the means herein disclosed, provided those stated by any of the following claims or their equivalents be employed.

We, therefore, particularly point out and distinctly claim as our invention:

1. In apparatus for hardening gears, an inductor surrounding a gear to be hardened, means for supplying high frequency current to said inductor, and a current conductor adapted to fill a portion of the space between adjacent teeth of the gear.

2. In apparatus for hardening a gear, an inductor in spaced relationship with a peripheral part of the gear, means for supplying current to said inductor, a conductor joining parts of adjacent teeth to provide a circumferential circuit through said parts of the teeth.

3. In apparatus of the character described, an inductor in spaced relationship with a surface having projections thereon, means for supplying current to said inductor, and a conductor adapted to conduct current peripherally of the said surface from one of the projections to another projection.

4. In apparatus of the character described, a conductor in spaced relationship wi h a surface As' quenching must be very rapid, it is having projections thereon, meansfor supplying current to said conductor, and a conductor adapted to carry current between projections, said last named conductor having a fusible portion, and said fusible portion having a part contacting with a part of the projections whereby current is caused to fiow across the part of the projection having contact with the last named conductor.

5. In a method of manufacturing a gear composed of material adapted to be hardened by heating and quenching, the steps comprising placing current conducting fillers between adjacent teeth of the gear, and causing heating current to flow thru said teeth and said fillers.

6. A method of hardening surfaces of a gear having internal and external teeth, comprising placing a conductor in contacting relationship with related internal teeth, placing a conductor in. contacting relationship with external'teeth, placing an inductor in spaced relationship with the internal teeth, placing an inductor in spaced relationship with the external teeth, and causing current to flow simultaneously in the inductors whereby heating currents are induced simultaneously in the internal and external teeth.

'7. In apparatus for heat treating teeth of a gear, supporting means for said gear, fillers adapted to serve as conductors of current between adjacent teeth 'of the gear, said fillers being loosely carried by a retractable support, an inductor surrounding a part of the gear, and means for establishing pressure contact between the teeth of the gear and the fillers.

8. In apparatus for heat treating teeth of a gear, a support for the gear, interconnected fillers adapted to be inserted in spaces between the teeth of the gear, an inductor adapted to surround said gear and said fillers, and means for supplying current to said inductor, the said fillers being adapted to melt at a temperature not less than the critical temperature of the gear teeth.

9. In apparatus for hardening gear teeth and the like, a multiplicity of current conductors, each lying between and having contact with two teeth of said gear and means to establish a heating current in a path through said conductors and said teeth peripherally of the gear.

10. In apparatus for hardening gear teeth and the like, a multiplicity of inter-connected conductors, each adapted to span the open portion between two of said gear teeth and contact said adjacent tooth surfaces and means for establishing a heating current in a path through said conductors and gear teeth peripherally of said gear.

11. In a method of heat treating outstanding FRANCIS S. DENNEEN. WILLIAM C. DUNN. 

